Precipitation Recovery of VO(OH)2 from Sodium Metavanadate Solution and Its Application in a Vanadium Redox Flow Battery

The vanadium redox flow battery has received intensive attention because of the increasing demand for safe energy storage systems. Generally, V2O5 is used for the preparation of a V electrolyte, but the synthesis of V2O5 from V leaching solution leads to significant environmental impact and requires complicated sequential reduction processes for the preparation of the V electrolyte. For sustainable V electrolyte production, we directly recovered VO(OH)2 from V leaching solution via a reduction–precipitation process using hydrazine (N2H4) as a reducing agent and used the VO(OH)2 to prepare a V electrolyte. When the V concentration was 1 M, the optimal conditions for the V recovery efficiency were a N2H4/V ratio of 1, pH 4, and a reaction time of 3 h. The effect of co-existing ions (Na, Mg, K, Fe, and Al) in the NaVO3 solution was also investigated. K exhibited the strongest adverse effect on the V recovery efficiency, and Fe and Al impurities were not removed from the recovered V compound by the washing process. A V electrolyte was prepared using the recovered VO(OH)2, and its charge–discharge performance was evaluated, revealing a high voltage efficiency (∼90.15% on average) and energy efficiency (∼86.79% on average) at a constant current density of 50 mA/cm2. These results demonstrate that directly using VO(OH)2 to prepare a V electrolyte is a simple and cost-effective process.